1. Field of the Invention
The present invention relates to a light guide plate for a surface light source device and a backlight unit using the same and, more particularly, to a light guide plate for a surface light source device and a backlight unit using the same, wherein a plurality of unit cells having microprism patterns engraved thereon is distributed and arranged on the back surface and a crossing angle between a microprism arrangement axis formed by a ridge direction of the microprism patterns on the unit cells and a lamp arrangement axis formed by the lamps, is disposed in an alternate angle.
2. Background of the Related Art
A light guide plate is chiefly applied to liquid crystal display devices and has been used as means for supplying light to a Liquid Crystal Display (LCD) module which is a passive element. That is, the light guide plate guides light emitted from light sources (e.g., LED lamps) disposed laterally to or under the light guide plate, diffuses the emitted light, and supplies the light to LCD modules stacked on the light emitting surface.
Furthermore, a backlight unit is formed by stacking a light diffusing sheet, a prism sheet, etc. on the side of a surface of the light guide plate from which light is emitted, thereby controlling the brightness and the viewing angle of the light.
Efforts have recently been made to reduce the number of parts by intergrating light guide plate and its perpheral parts to simplify the structure or reduce the prime cost.
For example, Patent Document 1 discloses a light guide plate in which a light scattering unit having a fine concave and convex portion is formed at the bottom of or any one of the surfaces of the light guide plate and to which a reflection plate for controlling the light scattering, such as transparent ink, is attached. According to this construction, a light guide plate having good sacttering efficiency and brightness can be fabricated simply and quickly. In this proposal, however, only brightness improvement by the light scattering unit and the reflection plate, not uniformity of brightness has been described. Furthermore, a manufacturing method of light guide plate by pattern shape molding of a depressed engraving on the back surface of the light guide plate is utilized in this proposal. The light guide plate manufactured by this method has slightly higher brightness than a light guide plate using a printing method. But, the problem is still continuing that brightness is degraded and the cost of a backlight unit is increased to the extent that a large quantity of optical sheets are required.
To solve the problems, the development of technology for significantly improving brightness by molding patterns, such as triangle structures, on the back of a light guide plate and controlling an optical path has been attempted. For example, Patent Document 2 discloses a light guide plate having a triangular reflection unit provided on the back of a light-emitting surface. This technology, however, is characterized in that brightness is excellent, but it has an irregular optical distribution in which the uniformity of brightness is insufficient because the triangular back reflection surface reflects light only in a limited and specific direction. This technology is also problematic in that a moire phenomenon due to a combination of prism shapes is generated when the prism is used as optical sheets on the top of the emitting surface.
Furthermore, since the LED lamps are used as light sources, a problem, such as brightness uniformity due to the formation of a dark portion between the LED lamps and lamps, and a problem, such as the generation of a moire phenomenon due to a combination of the prism structure at the back of the light guide plate and the prism sheet used as the optical sheet, continue to occur. To solve the problems, the inventors of the present invention have found that the moire phenomenon can be suppressed by improving a front brightness distribution if the ridge direction of the prism patterns formed on the back of the emitting surface or the microprism patterns formed on the back unit cells is disposed in a predetermined alternate angle to the incident direction of light in the light guide plate and thus have completed the present invention.